The present invention relates to an insulated wire suitable for use in a motor, particularly in a stator of a stator motor or a generator requiring a rigorous winding process.
For a generator (e.g. an alternator) or a motor for an automobile, or a compressor motor for a refrigerator, or the like, miniaturization and high-density configuration have progressed, while the needs for high performance and high power have grown in recent years. To meet these needs, it is necessary to insert additional, or larger-sized, conductors (i.e. electric wires) into a slot of an iron core of the insulated wire used in the stator side of these generators or motors. That is, to achieve this purpose, a more increased space factor (a ratio of the volume of an electric wire to the slot space) than ever before is strongly required. As a result, the insulated wire has generally been processed or worked on so rigorously or strongly that the shape of its cross section has become greatly altered, making it vulnerable to external breakage or damage, and thus decreasing conspicuously the reliability of the insulated wire itself. Accordingly, the development of an insulated wire whose insulation coating will not be broken even by external processing being so rigorous or strong that the conductor is deformed, and that has high resistance to external scratches, has long been desired.
On the other hand, many insulated wires coated with an electrically insulated material have been widely used as material for coils to be built in a variety of electrical machinery and apparatus. Recently, the process of coil winding using the insulated wire has been speeded up and rationalized and also switched from conventional hand-winding to automatic-winding using a coil winding machine. Further, the process of insertion of coils into a stator slot has been automated as well.
However, the automatic coil-winding process causes great stress on the coating of the insulated wire, due to high tension applied to the insulated wire, thereby inevitably making such insulated wires vulnerable to scratches or breakage. Moreover, the insulated wire has received more pressure by the insertion of coils into the stator slot due to the use of the machine, unlike in the case of conventional manual insertion. Under such circumstances, friction or rubbing is apt to occur frequently between wires, or between a wire and a substance contacted with the wire, readily causing insulation failure in coils.
Further, since miniaturization of the whole device and cost reduction can be realized by making the space factor of an insulated wire in a stator slot of a coil as large as possible, it is necessary to accordingly make the outer diameter of the wire smaller. In recent years, in the above-mentioned trend of making the diameter smaller, in order to increase the power of the device, efforts have been made to keep or increase the present diameter of a conductor, as well as to make the insulating coat thinner.
However, when a wire that has a smaller thickness of the insulating coat is utilized in the automatic coil-winding or the automatic insertion of a coil, in which the wire is used, into a stator slot, it results in an increased frequency of breakage of the coating, and an increased rate of occurrence of insulation failure of the coil.
To solve the problems described above, various methods have been proposed, including (1) reducing the coefficient of friction on the surface of an insulated wire, thereby reducing or preventing breakage caused by contact between insulated wires or between an insulated wire and another object, to suppress the occurrence of external scratches; (2) improving adhesion between an insulation coating and a conductor, thereby preventing the conductor from peeling off the insulation coating, and (3) improving the mechanical strength of the insulation coating, thereby increasing resistance of the insulation coating to breakage. The smaller the coefficient of friction, the easier the coil winding work. Also, the higher the mechanical strength of the coat, the lower the frequency of occurrence of breakage caused by the coil winding or the insertion of coils into a stator slot (hereinafter referred to as coil working, as a whole).
As the above conventional means, first, the methods (1) for reducing the coefficient of friction on the surface of an insulated wire are proposed to comprise the step of: applying, onto the insulated wire surface, a wax, an oil, a surfactant, a solid lubricant, and the like, as disclosed, for example, in JP-A-55-80208 (xe2x80x9cJP-Axe2x80x9d means unexamined Japanese patent application), JP-A-56-15511, JP-A-58-186107, and JP-A-61-269808; or using a coated/baked anti-friction material composed of a wax emulsifiable in water and a resin emulsifiable in water and to be set by heat, as disclosed, for example, in JP-A-62-200605; or carrying out lubrication by adding polyethylene fine powder or fluororesin fine powder to an insulation varnish itself, as disclosed, for example, in JP-A-63-119109 and JP-A-63-29412. The aim of the above methods is to protect an insulating layer from external scratches by surface slippage of an insulated wire, which protection results from the improved surface lubricity of the insulated wire.
Next, the method (2) described above for improving adhesion between an insulation coating and a conductor, by mixing an additive, adapted to improve the adhesion with a conductor, into a resin vanish, in order to improve work-resistance and abrasion-resistance of the insulated wire, thereby improving the resistance to external scratches, has been proposed, for example, in JP-A-56-143266 and JP-A-58-42672.
Further, the method (3) described above for improving the mechanical strength of the coat itself has been disclosed, for example, in JP-A-5-225830 and JP-A-6-196025, wherein, by taking the above (1) and (2) into consideration, the description that an insulation varnish, obtained by polycondensation of bitolylen diisocyanate and trimellitic anhydride, can improve the mechanical strength of the coating, has been provided. This method proposes, from the viewpoint of molecular structure of a resin, introducing many rigid structures into the molecule of a resin, thereby increasing the coating""s mechanical strength and reducing working scratches on the coating of the insulated wire. The unidirectional abrasion test (the test stipulated in JIS C 3003; a scratching test in which an electric wire is applied an increasing load, to scratch a coating thereof with a piano wire) of these insulated wires, which is recently considered to be an important coating breakage test, has revealed high quality as an insulated wire, and the insulated wire, even though it is made thinner, can prevent the coating from breakage at the time of coil working. However, because the level of softness of such insulated wires after being expanded, or after being subjected to a heat history, is lower than that of the conventional insulated wire, and because, especially when rigorous bending is applied, the insulated wires do not present sufficient softness, there is a risk that cracks or fractures may occur on the coating thereof.
Furthermore, even by using the above method (1) or (2), singly or in combination with each other, the current requirement for resistance to external scratches has not been satisfied. The insulated wires obtained even by using the above methods (1), (2), and (3) cannot provide sufficient external-scratch resistance required for overcoming various difficulties associated with winding work and insertion working of coils, and further improvement has long been desired.
In the insulated wire using an insulation coating with high mechanical strength obtained by employing the above method (3), a hard conductor having 0.2% yield stress ("sgr"0.2) of as high as about 120 to 140 MPa, has conventionally been used. Although less breakage at the time of coil-winding process may occur in such insulated wires composed of a coating of high mechanical strength and a conductor of high hardness, since sometimes the insulation coating suffers from breakage at the time of the insertion of coils into a stator slot, further improvement has been strongly required.
Accordingly, it is an object of the present invention to provide an insulated wire that, even if its insulation coating is made thinner, is excellent in resistance to scratches or damage, and that is able to prevent the coating from such breakage at the time of coil working, especially at the time of the insertion of a coil using the wire into a stator slot, under rigorous conditions.
Other and further objects, features, and advantages of the invention will appear more fully from the following description.